The present inventors had disclosed in Japanese Patent Laid-open Nos. 304393/1992 and 354890/1992 a method for purifying a bath liquid by electrolysis. This method employs an electrolytic cell provided with an ion-selective diaphragm between the anode and cathodes. According to this method, a bath liquid used for metal surface cleaning is supplied to the compartment between the anode and diaphragm (referred to as the anode compartment hereinafter). The bath liquid undergoes electrolysis such that metal cations contained therein migrate toward the cathode through the diaphragm by electrophoresis. Thus, metal cations are separated and collected in the cathode compartment, and finally they are removed by some means. This method would be of great industrial use if the catholyte to be circulated through the cathode compartment is controlled such that separated metal ions are converted into an easy-to-use form.
In contrast to the foregoing method, the present inventors propose a method for disposing of waste which occurs when a bath liquid used for metal surface cleaning is treated, instead of direct discharging, to avoid environmental pollution. This method employs an electrolytic cell provided with one or two ion-selective permeable diaphragms between the anode and cathode. According to this method, a used bath liquid is recycled to the anode compartment or the intermediate compartment between the diaphragms. It undergoes electrolysis such that metal ions migrate toward the cathode compartment through the permeable diaphragm by electrophoresis. The metal ions are changed into metal particles in the cathode compartment. Finally, the metal particles are removed by a magnetic separator, according to the present invention. This method offers the advantage of not only reducing the contaminant in the bath liquid but also recycling the metal component from the bath liquid.
The above-mentioned method for purifying a bath liquid suffers the disadvantage that the metal particles formed in the catholyte cannot be removed by a magnetic separator unless they are easily magnetized. For example, divalent (Fe.sup.+2) iron ions contained in a bath liquid gives rise to metal particles, such as triiron tetraoxide, which are easily magnetized and removed by a magnetic separator. By contrast, trivalent aluminum or chromium ions in a bath liquid do not give rise to metal particles which are easily magnetized